The invention relates to a silicone rubber material containing finely divided phase change materials and a process for producing such a material. The application of materials, which absorb, store and release large quantities of heat during a phase transition, into those materials which do not undergo such a phase transition within the same temperature range, leads to a thermo-regulating effect. This thermo-regulating effect can be used to enhance the thermal performance characteristics and the thermal comfort sensation of a variety of products such as sport garments, diving suits, protective garments, blinds, building materials, medical products, automotive products, etc. substantially.
Phase change material possesses the ability to change its physical state within a certain temperature range. When the melting temperature is obtained during a heating process, the phase change from the solid to the liquid state occurs. During this melting process, the phase change material absorbs and stores a large amount of latent heat. The temperature of the phase change material remains nearly constant during the entire process. In a cooling process of the phase change material, the stored heat is released into the environment in a certain temperature range, and a reverse phase change from the liquid to the solid state takes place. During this crystallization process, the temperature of the phase change material also remains constant. The high heat transfer during the melting process and the crystallization process, both without any temperature change, is responsible for the phase change material's appeal as a source of heat storage.
In order to contrast the amount of latent heat absorbed by a phase change material during the actual phase change with the amount of sensible heat in an ordinary heating process, the ice-water phase change process will be used. When ice melts, it absorbs an amount of latent heat of about 335 J/g. When the water is further heated, it absorbs a sensible heat of only 4 J/g while its temperature rises by one degree C. Therefore, the latent heat absorption during the phase change from ice into water is nearly 100 times higher than the sensible heat absorption during the heating process of water outside the phase change temperature range.
In addition to ice (water), more than 500 natural and synthetic phase change materials are known. These materials differ from one another in their phase change temperature ranges and their heat storage capacities.
Currently, only crystalline alkyl hydrocarbon phase change materials having different chain lengths are used for finishing yarns, textiles and foams. Characteristics of these phase change materials are summarized in Table 1.
TABLE 1Crystalline alkyl hydrocarbonsCrystallineMeltingCrystallizationLatentalkyltemperature,temperature,heat storagehydrocarbonsFormula° C.° C.capacity, J/gHeneicosaneC21H4440.535.9213EicosaneC20H4236.130.6247NonadecaneC19H4032.126.4222OctadecaneC18H3828.225.4244HeptadecaneC17H3621.716.5213HexadecaneC16H3416.712.2237
The crystalline alkyl hydrocarbons are either used in technical grades with a purity of approximately 95% or they are blended with one another in order to cover specific phase change temperature ranges. The crystalline alkyl hydrocarbons are nontoxic, noncorrosive, and nonhygroscopic. The thermal behavior of these phase change materials remains stable under permanent use. Crystalline alkyl hydrocarbons are byproducts of petroleum refining and, therefore, inexpensive.
Salt hydrates are alloys of inorganic salts and water. The most attractive properties of salt hydrates are the comparatively high latent heat values, the high thermal conductivities and the small volume change during melting. Salt hydrates often show an incongruent melting behaviour which results in a lack in reversible melting and freezing making them unsuitable for permanent use. Salt hydrates with reversible melting and freezing characteristics are summarized in Table 2.
TABLE 2Salt hydratesMeltingLatent heat storagetemperature,capacity,Salt hydrates° C.J/gCalcium Cloride Hexahydrate29.4170Lithium Nitrate Trihydrate29.9236Sodium Sulfate Decahydrate32.4253
In the present applications of the phase change material technology in textiles, only crystalline alkyl hydrocarbon are used which are microencapsulated, i.e., contained in small micro-spheres with diameters between 1 micron and 30 microns. These microcapsules with enclosed phase change material are applied to a textile matrix by incorporating them into acrylic fibers and polyurethane foams or by coating them onto textile surfaces.
U.S. Pat. No. 4,756,958 reports a fiber with integral micro-spheres filled with phase change material which has enhanced thermal properties at predetermined temperatures.
U.S. Pat. No. 5,366,801 describes a coating where micro-spheres filled with phase change material are incorporated into a coating compound which is then topically applied to fabric in order to enhance the thermal characteristics thereof.
U.S. Pat. No. 5,637,389 reports an insulating foam with improved thermal performance, wherein micro-spheres filled with phase change material are embedded.
The micro-encapsulation process of crystalline alkyl hydrocarbon phase change materials is a very time-consuming and complicated chemical process running over several stages making the microcapsules with enclosed phase change material very expensive.
In addition to the micro-encapsulation of phase change material, several attempts have been made to contain crystalline alkyl hydrocarbons as well as salt hydrates in certain macro-structures such as a silica powder, or a polyolefin matrix.
U.S. Pat. No. 5,106,520 describes a dry silica powder comprising phase change material.
U.S. Pat. No. 5,053,446 reports a polyolefin composition containing a phase change material and possesses enhanced thermal storage properties.
However, applications of these containment structures have shown that they are not providing a durable containment and the phase change material often disappears while in its liquid stage.